In the life cycle of human immunodeficiency virus (HIV)-1, assembly of the virion particle is an important step which is regulated by both viral and cellular factors (Demirov, 2004; Lopez-Verges, 2006). The HIV Gag protein is sufficient for assembly, budding and release from the host cell of virus-like particles (VLPs). Each particle is enveloped by a lipid bilayer derived from the host cell; and the envelope glycoprotein (Env) is incorporated into the particle during the process of assembly (Deml, 1997; Yao, 2000). The Gag has a “late” (L) domain that promotes particle release by interacting with components of the cellular endosomal sorting pathway (Freed, 2002). Gag is also post-translationally modified with an N-terminal myristate group, which is thought to target Gag to lipid rafts thus aiding in assembly (Provitera, 2006).
It has been reported that the transmembrane (TM) and cytoplasmic tail (CT) domains of gp41 exert a key role in incorporation of the HIV-1 envelope glycoprotein (Env) during HIV assembly. The TM and CT domains of HIV-1 and SIV Env have important effects on the orientation, surface expression, surface stability and Env incorporation into particles (Zingler, 1993; Vzorov, 2000; Ye, 2004). Previous studies suggest that specific regions in Env are involved in the interaction with Gag in assembly (Lopez-Verges, 2006; Demirov, 2004); however, the detailed mechanisms that determine the incorporation of Env into VLPs remain to be determined. It is also not well understood whether different viral core proteins have preferences for their cognate Env or whether heterologous CT/TM-CT sequences prefer a specific matrix protein for assembly into VLPs.
In early studies, it was observed that HIV-1 Env is expressed and secreted very inefficiently in various expression systems including yeast (Barr, 1987) and mammalian cells (Lasky, 1986; Chakrabarti, 1986; Kieny, 1986). The signal sequence is important in directing Env to the endoplasmic reticulum and eventually to the cell surface. The substitution of the HIV Env signal peptide (SP) with that from honeybee mellitin was shown to promote higher level expression and secretion of HIV-1 gp120 (Li, 1994). HIV-1 Env also has a CT sequence with over 150 amino acids (aa) whereas glycoproteins of other viruses including MMTV, Lassa fever virus (LFV), BV gp64, and influenza virus HA have much shorter CT sequences between 7 to 43 aa in length. Interestingly, these viruses with shorter CT sequences incorporate their glycoprotein into virions at much higher levels than those in HIV-1 (Compans, 1978).